In recent years, nitride semiconductor materials have become of interest as materials for producing a semiconductor light-emitting device which emits light of short wavelength. Generally, a nitride semiconductor is grown on a substrate (e.g., oxide crystals such as a sapphire single crystal or Group III-V compound semiconductor crystals) through a method such as metalorganic chemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE), thereby forming an n-type layer, a light-emitting layer, and a p-type layer, which are stacked on the substrate.
At present, among these methods, metalorganic chemical vapor deposition (MOCVD) is most widely employed in the industry as a method for growing compound semiconductor crystals. In MOCVD, an organometallic compound serving as a Group III source gas is fed, along with a Group V source gas into a reactor tube in which a substrate such as sapphire, SiC, GaN, or AlN is placed, and crystal growth is performed at about 700° C. to about 1,200° C., to thereby form an n-type layer, a light-emitting layer, and a p-type layer.
After completion of growth of these semiconductor layers, a negative electrode is formed on the substrate or the n-type layer, and a positive electrode is formed on the p-type layer, whereby a light-emitting device is provided.
Conventionally, the light-emitting layer employs InGaN having a composition regulated for emitting light of a desired wavelength. When the InGaN layer is sandwiched by layers having wider band gaps, a light-emitting layer of a double-heterojunction structure is produced. Alternatively, a light-emitting layer of a multiple quantum well structure is produced from the InGaN layer on the basis of a quantum well effect.
Conventionally known multiple quantum well structures include a quantum well structure of InGaN—GaN included in a Group III-V (GaN-based) light-emitting device (F. Scholz et al., “Investigation on Structural Properties of GaInN—GaN Multi Quantum Well Structures,” Phys. Stat. Sol. (a), Vol. 180, (2000), p. 315). The process for growing the quantum well structure includes maintaining a substrate at 1,000° C., forming a barrier layer at high temperature, lowering the substrate temperature, maintaining the substrate at the lowered temperature, and forming an InGaN well layer.
Alternating formation of the barrier layer and the well layer is repeated, to thereby form a light-emitting layer.
In a known variation of the aforementioned technique, the barrier layers are grown with elevating temperature, the barrier layers are stacked at high speed, and nitrogen serving as a carrier gas is replaced by hydrogen (Japanese Patent Application laid-Open (kokai) No. 2002-43618). The technique suitably attains enhancement of emission efficiency, reduction of production cost by shortening the time of forming the light-emitting layer, prevention of sublimation of In through growth with temperature elevation, and other effects.
The light-emitting devices fabricated through stacking of the layers based on any of the aforementioned methods have the problem that reverse withstand voltage (i.e., the absolute value of the voltage required for inducing flow of a 10 μA current in the reverse direction in a light-emitting device having a P—N junction) is deteriorated as time elapses in an aging process.
Specifically, a current of 30 mA is caused to flow in the forward direction in each light-emitting device, and reverse withstand voltage of the light-emitting device is measured before and after the device is allowed to stand for a predetermined period of time. In the experiments described in the Examples of the present specification, the reverse withstand voltage is measured after maintenance of 0 hour, 20 hours, and 40 hours.
The light-emitting devices fabricated through stacking layers on the basis of the above-disclosed conventional techniques fail to attain a desired emission strength. Thus, there is a demand for further enhancement of emission efficiency.